The present invention relates to the field of semiconductor devices, and, more particularly, to a polymeric composition for packaging an electronic semiconductor device. Moreover, the present invention also relates to a plastic packaging material for microelectronic applications, which may be obtained from such polymeric composition, and to a semiconductor device including such packaging material.
In the field of microelectronics, it is quite common to encapsulate electronic semiconductor devices, such as power metal oxide semiconductor (MOS) devices, within packages of plastic material. Power MOS electronic devices include a plurality of layers having different chemical structures (e.g., dissipating elements, support frames, die made from semiconductor materials, and plastic packaging materials) whose compatibility determines the performance and reliability of the device.
The reliability of a power device may be measured by a number of tests. In particular, the so-called High Temperature Reverse Bias (HTRB) test allows reliability to be estimated by subjecting the device to high temperatures during reverse biasing. The behavior of the device in this test depends upon the physical-chemical conditions of the circuit die and the interactions with the packaging material.
Usually, plastic packaging material is produced by hardening a polymeric composition including a thermosetting resin and various additives, such as reinforcing fillers based on fused or crystalline silica, for example, and at least one control agent for the rheology of the polymeric composition (generally based on siloxanes). The thermosetting resin usually includes an epoxy resin that is typically obtained from an epoxy pre-cured with phenolic resin or an epoxy pre-cured with an anhydride.
It has been suggested that to have a low ionic content and a high volume resistivity the bulk characteristics of the plastic packaging material of a semiconductor device may require alteration. For example, it has been suggested that the amount of ions, among which Na+ and Brxe2x88x92 ions come from raw materials, be reduced to provide a high volume resistivity, preferably higher than 1xc3x971012 xcexa9cm. However, the reliability value obtained by subjecting semiconductor devices of the prior art to the HTRB test is inadequate compared to the ever increasing reliability level required from such devices, particularly from power devices.
An object of the invention is to provide a polymeric composition for packaging a semiconductor electronic device that provides improved reliability of the semiconductor device with respect to prior art devices.
Applicants have determined that to address the above problem it will not suffice simply to alter the bulk characteristics of the die packaging to provide low ionic concentration and a high volume resistivity. Rather, it may be necessary to reduce the polarity of the plastic packaging material layer as much as possible at the interface with the die itself. In fact, applicants have found that the polarity characteristics at the interface between adjoining layers significantly affect the mechanical and electrical performance of the device.
According to the invention, the above technical problem is solved by a polymeric composition including at least one epoxy resin, at least one curing agent in an amount between 30 and 110 parts by weight per 100 parts by weight of epoxy resin, at least one silica-based reinforcing filler in an amount between 300 and 2300 parts by weight per 100 parts by weight of epoxy resin, and at least one control agent for the rheology of the polymeric composition. The at least one control agent may be substantially free from polar groups and may be present in an amount between 0.1 and 50 parts by weight per 100 parts by weight of epoxy resin.
From tests conducted by applicants, the electrical behavior of the interface plastic package material and the semiconductor material die was found to be substantially correlated to the quantity of polar groups existing at the interface. In particular, it was found that a reduction in the quantity of polar groups at the interface, in particular those coming from the control agent for the rheology, provides significant improvement in the reliability of the semiconductor device.
The at least one epoxy resin may be selected from the group including bisphenol A type epoxy resin, phenol-novolac type epoxy resin, creosol-novolac type epoxy resin, glycidyl ester type epoxy resin, biphenyl type epoxy resin, polyfunctional epoxy resin, glycidyl amine type epoxy resin, linear aliphatic epoxy resin, alicyclic epoxy resin, heterocyclic epoxy resin, halogenated epoxy resin, and mixtures thereof. The chemical structures of these resins may be seen in FIGS. 1A to 1L.
Additionally, the epoxy resin may include a concentration of chlorine ions lower than 10 ppm and a concentration of chlorine in hydrolyzed form lower than 0.1% by weight, because such ions may cause electronic failures. Preferred epoxy resins include those of the glycidyl ester type and novolac type epoxy resins, the novolac type including 170 to 300 epoxy equivalents. These provide good workability during molding and also good electronic reliability.
The at least one curing agent may be a phenolic resin selected from the group including phenol-novolac resin, cresol-novolac resin, bisphenol A resin, phenol-aralkyl resin, dicyclopentane phenolic resin, bisphenyl type phenolic resin, polyfunctional phenolic resin, other denatured phenolic resins, and mixtures thereof. The chemical structures of these resins may be seen in FIGS. 2A to 2F.
Advantageously, the epoxy resin and the phenolic resin may be mixed so that the ratio between the number of epoxy equivalents of the epoxy resin and the number of equivalents of the hydroxyl groups of the phenolic resin is between 0.5 and 1.5. It has been determined that if this ratio exceeds the above defined range, the mechanical strength of the cured epoxy resin may be reduced.
The silica-based reinforcing filler may include fused silica powder or crystalline silica. The silica powder may generally assume a spherical, lumpy or fibrous shape. Spherical silica is generally used in combination with other fillers which have non-uniform diameters. The silica-based reinforcing filler is incorporated in the polymeric composition to adequately protect the semiconductor device and to impart improved workability to the polymeric packaging composition during molding. This, in turn, reduces strain in the device itself.
The silica-based reinforcing filler may include spherical silica in an amount between 0.05 and 20% by weight of the total amount of silica-based reinforcing filler. Advantageously, spherical silica provides improved flow of the polymeric composition. In particular, spherical silica reduces the so-called resin-flush phenomenon, i.e., when part of the polymeric packaging composition comes out from the air spaces between the various sections of the mold. The spherical silica preferably has an average particle diameter between 0.3 and 1.5 xcexcm and a surface area between 3 and 10 m2gxe2x88x921.
In particular, it was found that if the surface area value falls below 2 m2g1, the so-called resin-flush phenomenon cannot be reduced sufficiently. On the other hand, if the surface area value exceeds 10m2gxe2x88x921, an undesired moisture absorption by the polymeric composition may take place.
Furthermore, the polymeric composition of the invention may further include at least one silica coupling agent suitable for reacting with the surface hydroxyl groups of silica. Advantageously, the coupling agent performs the function of coupling the silica-based reinforcing filler to the polymeric matrix upon prior reaction with the epoxy polar groups including the same. In this way, as the bond takes place in corresponding polar groups existing in the polymeric matrix, the number of free epoxy groups, and decreases further reduction in the polarity of the polymeric composition at the interface with the semiconductive material die ensues. This results in a further advantageous increase in the reliability of the device.
By way of example, the silica-coupling agent may be an amino-silane agent including secondary amino groups having the following structural formula:
R-NHRxe2x80x2Si(ORxe2x80x3)3xe2x80x83xe2x80x83(I) 
where:
R=Phxe2x80x94, CnH2n+1, n=1, 2, . . .
Rxe2x80x2=xe2x80x94(CH2)mxe2x80x94, m1, 2, . . .
Rxe2x80x3=CkH2k+1, k=1, 2, . . . .
The secondary amino groups of the amino-silane agent do not significantly increase the polarity of the polymeric packaging composition at the interface with the semiconductor material die, which again results in device reliability. The amino-silane agent may be selected from the group including xcex3-N-methyl amino-propyl triethoxy silane, xcex3-N-ethyl amino-propyl triethoxy silane, xcex3-N-phenyl amino-propyl triethoxy silane, xcex3-N-phenyl amino-propyl trimethoxy silane, N,N-bis [methylidiethoxysilyl)propyl]amine, N,N-bis[xcex3-(trimethoxysilyl)propyl]amine, and mixtures thereof. Moreover, the amino-silane agent may be included in an amount between 0.05 and 3% by weight of the total amount of silica-based reinforcing filler.
In fact, it has been observed that when the amount of amino-silane coupling agent falls below 0.05% by weight of the total amount of the silica-based reinforcing filler, silica is not sufficiently coupled to the polymeric matrix. As a result, the packaging has poor homogeneity and is fragile. Further, along with the resin-flush phenomenon a so-called resin-bleeding may appear, i.e., resin and silica-based reinforcing filler coming out from the air spaces of the mold, resulting in undesired product loss. In this condition, silica may even show a particular moisture permeability.
Conversely, when the amount of amino-silane coupling agent exceeds 3% by weight of the total amount of silica-based reinforcing filler, the portion of the amino-silane coupling agent that has not reacted is segregated at the interface. This may cause, on the one hand, an increase in polar groups corresponding to the interface itself and, on the other hand, dirtying of mold surfaces.
The control agent for the rheology of the polymeric composition may be substantially free from polar groups. Preferably, the control agent for the rheology of the polymeric composition is polydimethylsiloxane substantially free from polyoxyalkylenether groups having the following structural formula:
[Si(CO3)2xe2x80x94O]nxe2x80x83xe2x80x83(II) 
a viscosity of which at 25xc2x0 C. is between 5 and 106 mm2/s. Advantageously, polydimethylsiloxane acts effectively on the rheology of the polymeric composition and can improve the workability of the polymeric composition. The control agent for the rheology of the polymeric composition may be added in an amount between 1 and 5% by weight to the total amount of amino-silane coupling agent.
The polymeric composition may further include ingredients known in the art such as catalysts (e.g., imidasole and derivatives thereof, derivatives of tertiary amines, phosphines or derivatives thereof), releasing agents (e.g., natural waxes, synthetic waxes, metal salts of linear aliphatic acids, esters or acid amides and paraffins), flame-retardant agents (e.g., bromotoluene, hexabromobenzene and antimony oxide, coloring agents such as carbon black), impact modifiers (e.g., silicone rubber and butadiene rubber), and other common additives. Further, the polymeric composition of the invention may be prepared according to conventional mixing operations using mixing apparatuses also known in the art. At the end of the conventional mixing operations, the polymeric composition is cooled, ground to a powder, and then pressed into pellet form.
Plastic packaging material for microelectronic applications is also provided according to the invention. The plastic packaging material may be obtained by molding and curing the above described polymeric composition.
Encapsulation of a semiconductor device may be carried out by submitting the polymeric composition to conventional molding operations, such as compression molding, transfer molding, or injection molding, for example. Preferably, the molding operations are performed by a low-pressure transfer molding technique. The molding operation may be performed in steps. The first step may include injecting a molding compound inside the mold for two minutes, during which the vast amount of the polymer is cured. The second step includes post mold curing in oven at 150-190xc2x0 C. in N2 completing the curing process.
A semiconductor electronic device is also provided according to the invention and includes at least one electronic circuit mounted on a frame and a package of plastic material on the electronic circuit. The package may be obtained by molding and hardening the above described polymeric composition. Semiconductor electronic devices according to the invention may include, but are not limited to, power devices or power packages, integrated circuits (IC), large scale integrated circuits (LSI), bipolar devices and MOS, thyristor devices, diodes and memory devices.